For numerous applications in industry, laboratories, military and commerce, it is highly desirable that there be a supply of A.C. power of a relatively constant frequency, of a relatively constant selected voltage and that this power be furnished without interruptions. A marked drop in frequency, a large drop in voltage or a complete interruption can cause either malfunctioning or misfunctioning of the apparatus to which the A.C. power is being furnished. In some cases serious damage may result. Computers and electronic control systems can malfunction seriously if the voltage or frequency depart from narrowly restricted limits.
Public utility systems are regarded as being quite reliable, but as is brought out in an article on pages 100 to 102 in the May 10, 1975 issue of "Electronic Design", in the average utility system, major line faults with voltage drops of 75% or more, including complete outages, lasting for relatively long periods of time, can be expected about 10 times a year; while minor faults during which the voltage may drop as much as 25% occur on an average of from 500 to 1000 times a year. In such systems, transients and spikes in the voltage in the utility lines may average more than 10,000 times per year, with line surges being up to as much as 2,000% of normal line voltage and lasting for periods of up to several milliseconds and sometimes exceeding 10 milliseconds. It is common knowledge that extremes of cold or hot weather place such demands on the public utility lines that the utility may be forced to reduce line voltages up to as much as 10% (so-called "brown-outs") or to eliminate delivery of power to certain sections or to isolate industrial plants from power. Unexpected or unpredictable events such as accidents, storms, fires, and the like cause circuit breakers to function to isolate the areas where lines may be shorted or damaged by such occurrences.
As a broad categorization, the probability of power disturbances in, for example, 60 and 50 Hz utility power lines, are as follows:
Each of these three classes of power line disturbances can cause serious problems in the functioning of computers, electrical control equipment such as numerically controlled machine tools, communications including radio, TV broadcasting and telephony, process controls in shops and laboratories, and detection and monitoring systems. Even a brief interruption or a voltage spike or drop beyond a rather narrow margin can result in a malfunction of a computer, for instance, which is often not observed or noted, and great difficulty is encountered in subsequently determining that a malfunction did occur and where or when the erroneous operation took place. When process controls fail to function due to power failure they may result in a dangerous or costly malfunction of the controlled apparatus with damage not only to the apparatus being controlled, but also to the material or product being treated or processed.
It is known to provide stand-by or auxiliary power sources in hospitals, radio and television stations, and police stations, for instance, to furnish emergency power in the event that power is interrupted in a utility line which normally supplies the power needed in these facilities. Two of the most widely used stand-by systems are (1) storage batteries with electrical controls such as D.C. to A.C. inverters and suitable switching means, and (2) engine operated generators with controls and switches. The shortcomings of the battery systems are their short periods of usefulness which are of the order of 15 minutes for full power replacement, long recharging times after each use of up to from 8 to 20 hours, high first cost, and substantial space and maintenance requirements. Engine-generator units take a material time period to get them started and to deliver power, so that normally there is a substantial time interval from the instant when the utility power is interrupted until the controls are activated and the engine is started, and auxiliary power is supplied to the equipment. The A.C. power from such stand-by generators is usually not comparable to the normal utility A.C. power in evenness of voltage levels and regularity of frequency. In all cases, it is extremely costly to provide any auxiliary system that will take over immediately upon the failure of the utility power so as to furnish A.C. of high uniformity, of precisely selected frequency and a substantially constant selected voltage. A recent survey of systems for protecting electrical apparatus from the effects of various power line failures is set forth in an article entitled "Power Protection Equipment: A Survey" appearing on pages 38 to 45 of the July 1977 issue of "Mini-Micro Systems". The capabilities and the shortcomings of many such commercially usable systems is set forth therein. It should be noted that an error appears on page 41, in that in the horizontal two rows under "Noise Rejection", the "Roesel Generator" (U.S. Pat. No. 3,521,149) should be indicated as "superior rejection", rather than being listed under "none".
It is highly desirable to have available a non-interruptible power system (NIPS) to provide polyphase A.C. of a substantially constant selected frequency and a substantially constant selected voltage for many commercial, industrial, governmental and defense, scientific and medical applications at a low original cost, that is efficient, easy and inexpensive to maintain, and is highly reliable. It should be compatible or readily phased in with a public utility power system.